`ConstDensityThermo::standardConcentration(k)` is now calculated
as `density()/molecularWeight(k)`, rather than the previously
incorrect `molarDensity()`.
Note that this causes a problem for any species where
`molecularWeight(k)=0` (i.e. vacancies). Such species should be
avoided, in this phase model.
For that reason, `sofc-test.xml` is changed so that the oxide bulk
is modeled as an `IdealSolidSolution`
